The present invention relates to a method of measuring and controlling the level of an elevated temperature liquid in a container, and particularly to a method of measuring and controlling the level of molten metal in a vessel, for example, a continuous casting mold.
A system commonly used to measure the level of molten steel in a continuous caster mold utilizes a plurality of thermocouple probes located at uniformly spaced elevations on the mold wall. The probes provide electrical voltage signal outputs corresponding to the thermal profile along the mold wall caused by the liquid steel in the mold. The conventional apparatus for determining the liquid level from the probe output signals is shown in U.S. Pat. No. 3,204,460, Milnes and U.S. Pat. No. 3,399,566, Wilson. These recorder balance slide-wire systems have worked adequately for many years. But, recent changes in casting practices and more stringent process control requirements have established the need for changes in this equipment. Variation in the casting flux and flux buildup on the mold walls, as well as submerged tube pouring practices, have increased the problems associated with determining the level of steel in the mold. Also, false level indications sometimes occur due to sudden mold level and casting speed variations. Perhaps the most common problems are false recorder oscillation and sluggish response due to improper amplifier gain adjustments. Serious consequences can result from poor probe contact or shorted probes.
Recently, a digital method of analyzing the probe signals and determining the liquid level was devised in order to overcome the above-mentioned problems. This method is the subject of a prior application of Wilson and Laycak, commonly assigned with the present application. This prior digital method has been subject to spiking or jumps in the indicated level whenever the indicated level passes through a probe location. This occurs more frequently of course when the desired level in the mold happens to be located at the location of one of the probes instead of between probes. The present application is an improvement in the prior digital method which is designed to overcome the problems just mentioned.